1. Field of the Invention
The present invention relates to a process for the preparation of complex oxide powders which are used broadly as catalysts, gas electrodes and the like. More particularly, the present invention relates to a process for the preparation of mixed oxide powders which are very fine and highly bulky and have a large specific surface area.
2. Brief Description of the Prior Art:
A material composed of at least two metal oxides is generally called a complex oxide. Materials formed by merely mixing metal oxides cannot be used as catalysts or gas electrode but only materials in which these metal oxides form a solid solution can be applied to these uses. In the case of a solid solution, a stoichiometric relation is established among the numbers of the constituent metals and oxygen. By the term "complex oxide" used in the instant specification is meant an oxide including at least two metals as the constituent elements in which these metal oxides form a solid solution.
Various useful complex oxides are known in the art. For example, a material represented by the following general formula: EQU Ln.sub.2.sub.+x M'.sub.x Cu.sub.1.sub.+y M".sub.y O.sub.4
wherein M' stands for Y, Ln is at least one element selected from rare earth elements having an atomic number of 59 to 71 and alkaline earth metals, M" is at least one element selected from Li, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Zr, Sn, Ce, Mo and W, and x and y are numbers satisfying the requirements 0.ltoreq.x.ltoreq.2 and 0.ltoreq.y.ltoreq.1,
Is a mixed oxide having a K.sub.2 MgF.sub.4 crystal structure, and this material has very excellent properties as a catalyst for disposal of exhaust gases.
Further, it is known that a material represented by the following general formula: EQU M.sub.1.sub.+x M".sub.x M'".sub.3.sub.+y
wherein A is at least one element selected from Y and rare earth elements having an atomic number of 57 to 71, M" is at least one element selected from alkaline earth metals, M'" is at least one element selected from transition metal elements, and x and y are numbers satisfying the requirements 0.ltoreq.x.ltoreq.1 and 0.ltoreq.y.ltoreq.0.5; is a complex oxide having a perovskite crystal structure, and a porous electrode formed by using this material has very excellent properties.
As typical instances of other well-known complex oxides, there can be mentioned ferrite, garnet, lead zirconate, lead titanate, barium titanate, and lead zircotitanate.
These various complex oxides are used broadly in various fields.
Complex oxides have heretofore been prepared ordinarily by the coprecipitation method, the freeze-drying method and other methods.
According to the coprecipitating method, constituent metal elements are coprecipitated from a solution of salts of metal elements of an intended complex oxide such as nitrates, acetates, oxalates and chlorides, and the coprecipitate is separated from the liquid and heat-decomposed, and the decomposition product is heated so as to obtain the intended complex oxode powder having a uniform composition.
This is one of methods heretofore used most frequently for the preparation of complex oxide powder, but this method is defective in that various additional steps such as the step of washing the coprecipitate are required and the bulkiness of the resulting complex oxide powder is low and the specific surface area is small.
According to the freeze-drying method, the temperature of a solution of salts of the constituent metal elements such as mentioned above is rapidly lowered to freeze the solvent and separate the salts from the solvent in vacuum, and the separated salts are heated as in the coprecipitation method.
This method is advantageous in that a complex oxide powder having a very large specific surface area can be obtained, but the method is defective in that the process steps are complicated and the bulkiness of the resulting complex oxide powder is low. Accordingly, the application of this method is limited.
Furthermore, there have been proposed a method comprising mixing uniform powders of salts of the constituent metal elements and heating the resulting homogeneous mixture and a method comprising evaporating a solution of salts such as mentioned above to dryness to separate the salts from the solvent. However, these methods are defective in that the process steps are complicated and the properties of the resulting complex oxide powder are undesirable. None of these conventional methods are satisfactory from the industrial viewpoint.